This invention relates to cross-linking of a fluorine-containing graft copolymer by exposure to ionizing radiation after forming the copolymer into a desired shape.
For rubbers and also for some other types of polymers, cross-linking is an important means of improving or desirably modifying the physical properties. Usually cross-linking is accomplished by using a cross-linking agent. For example, polyamine, peroxide or polyol is used for cross-linking of fluororubbers.
It is known that some polymers can be cross-linked by ionizing radiation. In short, cross-linking reaction takes place if suitable free radicals are produced in the chain of the irradiated polymer and if these radicals combine with one another at a relatively high rate. This method is important for thermoplastic polymers and is industrially employed for producing cross-linked polyethylene.
Furthermore, researches have been made on the radiation-induced cross-linking of some fluororubbers and fluorocarbon resins. For example, Japanese patent application publication No. 48-38465 shows that tetrafluoroethylene/propylene copolymer rubber can be cross-linked at a relatively low exposure dose by adding an allyl compound to the rubber, though the addition of such a compound is not an indispensable requisite for radiation-induced cross-linking, and U.S. Pat. No. 3,947,525 shows that the addition of an allyl compound to a fluorocarbon resin is effective in enhancing the tensile strength and thermal properties by irradiation. Japanese patent application provisional publication No. 59-62635 relates to radiation-induced cross-linking of a thermoplastic fluororubber after moulding and shows that in this case the addition of an allyl compound is not appreciably effective and that a sufficient effect of irradiation is obtained without adding such a compound in advance.
Meanwhile, fluorine-containing graft copolymers are attracting increasing interest since such copolymers are capable of uniting the properties characteristic of a fluorocarbon resin and flexibility or elasticity of a fluororubber. Typical examples of promising fluorine-containing graft copolymers are ones shown in U.S. Pat. No. 4,472,557, which can be formed into various shapes and will have wide uses. More particularly, the disclosure of this patent includes elastic copolymers obtained by graft copolymerization of a fluorine-containing monomer that gives a crystalline polymer, such as vinylidene fluoride, with a basic fluoroelastomer which is a copolymer having peroxy bonds in the polymer chain. The grafting is accomplished by thermal decomposition of the peroxy bonds. These fluorine-containing graft copolymers are excellent in mechanical properties, chemical resistance and mouldability. However, when substituting these graft copolymers for conventional fluororubbers sometimes it is desirable to improve the high temperature characteristics and mechanical strength of the graft copolymers.